Oratories



July 3, 1928.

` L/KM. HULL umaon op Ann MEANS Fon ADJusTING Aunrow cmdums Filed July14, 1925 I I I r l I I l I L (i KT;

1 I I I l I I l I I l I n I A I 0mm/r L @mind/S INVENTOR: 06W M BY ,l

ATTORNEYS:

6 W .7 f L 1I Ilii. A

Race/'viar- Patented July 3, 1928.

UNITED ,STATES ATENT FFICE..

LEV/IS M. HULL, OF BGONTON, NEW JERSEY, ASSCTNOR TO RADIO FREQUENCY LAB-OBATORIES, INCORPORATED, 0F BGONTON, NE'W JERSEY, A CORPORATION OF NEWJERSEY.

METHOD OF AND MEANS FOR Application tiled July 14;,

lily invention i: etes to audion repeater circuits and in particular toa method oli and means for adjusting such circuits to be unidirectionalin action.

l use the term audion repeater circuit to include any electrical networkcomprising one or more audion tubes so connected that electricaloscillations may be received by an input branch and delivered inamplified or rectified term to an output branch ot said circuit. By anaudion repeater` circuit which is unidirectional in action 'I@ mean anaudion repeater circuit in which means are provided tor diminishing oreliminating elilects upon the input circuit terminals due to potentialvariations in the output branch.

It is well known that the action ot audion repeater circuits, and inparticular ot audion amplifier circuits, is not ordinarily evenapproximately unidirectional; The absence o't unidirectional action isdue to a number ot causes, prominent among which are the teodback otcurrents through the inherent audion tube capacities, and theretroaction duc to electrostatic coupling between audion stages when anumber oit such stages are cascaded. In the copendiner application crtStuart Ballantine, Serial No. 728,703, tiled June 1.7, 1924, aredescribed the precautions which it is necessary to taire inv order tomalte an audion repeater stage practically unidirectional in action.

rIhere are a` number ot circuits capable oit providing unidirectionalaction or some approximatiou thereto. One example ot auch a circuit isthat co'lnmerically known as the neutrodyne. Another example ol such acircuit is that .shown in Figure I. ln all eircuits ot' this type it iscustoniary to provide certain lined circuit elements and one or moreVariable circuit elements, and' to adjust said variable element orelements to pro vide the desired unidirectional etlgect. This adjustmentis usually a manutacturei"s adjustment which necessary because ofcoinmercial variations `in the circuit elements and audion tubes. It thecircuit is of the lVheatstone bridge class, in which input and outputbranches `term conjugate arms ot a bridge, this process of adjustmentusually comprises balancing the bridge. My invention concerns suchadjustment oit circuit elements of an audion repeater circuit which isADJUSTING ADION CIRCUITS.

1925. Serial N0. 43,533.

capable oit' being made unidirectional in action, and it comprisesadjusting such circuit elements under the operating conditions ot theaudion repeater circuit.

Referring now to the drawings, F ig. I is a circuit diagram ot an audionamplilier stage which may be made unidirectional in action. F Il is adiagran'imatic representation of apparatus arranged according to myinvention. .ll ig. HI is an alternative arrangement of apparatusaccording to my uirention. i

l shall first describe the circuit ot Fig. I. ln this circuit theimpedai'ice network ot the audion amplifier .stage is arranged in. theforni oi an alternating current liVheatstone bridge in which the inputcircuit et the amplitier stage and the output circuit ot the amplilierstage, which coincides with the output circuit oi the audion tube, termconjugate arms. These circuits are thus electrically isolated itroin oneanother when the bridge is balanced, and potential variations :in theoutput circuit impedance are prevented from affecting the input circuitterminals. This circuit is disclosed and claimed in an application ot'Stuart Ballantine, tiled April 3, A1.923, Serial No. 629,702.

@ne balancing arm ot' the lVheatstone bridge is the inherent capacity ofthe audion tube represented by the dotted lines at Cm. Another balancingarm is the capacity Cc between the grid and .tilament of the tube.rl'his capacity may be the inherent grid-Fila ment capacity ot the tube,as represented at Cf; by the dotted lines, or it may be this inherenttube capacity in parallel with an eX- ternal capacity here shown at C,ior the salte ot generality. The third arm o'f the bridge is the portionLof` ot the coil L, and the tourt-li arm is composed oi iuductances L7)and il., and the mut-ual inductauce Mw. The lower terminal 2 of theinput circuit is brought by means 0f the mutual inductance lil23 to thesame alternating current potential as anr intermediate point X in theplate circuit impedance.

G and 2 are the input terminals of the amplifier stage and the remainingpair ot conjugate terminals of the lVheatstone bridge are the points Pand F. These points are, therefore, possible output terminals of thestage. I find it convenient, however, in cases Where a. tuned outputcircuit is desirable, to provide this circuit as an extra resonant meshcomprising coil L0 and condenser CO, which is coupled to the Whole otthe coil L2 through mut-ual inductance M20. The output-circuit'load thenhas the nature ot the parallel impedance oit this tuned circuit and itis immatei'ial for the purpose ot my invention Whether I use the pointsP and F or the terminals of the coil L0 output terminals. lt is commonpractice, however, when such a stage is coupled to a succeeding stage,to employ as output terminals the points so designated in Fig. l.

In order to render such an amplifier stage susceptible of adjustmenttobe unidirectional in actionr the circuital restrictions enumerated inthe above mentioned co-pending application, Serial `blo. 728,708, etStuart Ballantine should be observed. r1his ampliiier stage toi-ins nopart, however, ot the present" invention. lt has been described here asay typical form ol circuit to the adjustment of which my invention isapplicablc.

It is custom ary in the above circuit to make C, the variable element,by the adjustment ot which the YVheatstone bridge is balanced. It hashitherto been common practice to make this adjustment, or the equivalentad,- justment in circuits o'l other types, when the normal spacecurrents ol the audion or audions are not present. ylChat is, the audionor audions have not been in the normal operating condition when suchadjust-niente; were made. My investigations have disclosed in certainwidely-used commercial audions. when the space currents are ilo'wing, achange in the electrostatic capacity between the grid and plateelectrodes ot as much as 50 percent ot its value when the tube is cold;but prior to my invention no process for positively ascertaining thecondition ot balance when the space currents are flowing has beenavailable. rlhis results in an imbalance or inaccurate adjustment whenthe tube is operating. This unbalancc has hitherto been neglected andhas consequently resulted in regenerative or retroactive effects. ln thecase ot a circuit which is capable ot being completely balanced suchcomplete balance has theretore not hitherto been obtained, While in thecase of a circuit which can at best only be neutralized or compensatedfor certain frequencies this neutralization or compensation has hithertofallen short ot the optimum.

My method ot ascertaining when the proper circuit adjustment is obtainedcomprises in general impressing` an oscillatory or alternating voltageupon the output branch of the audion repeater circuit. In all previouslmethods et'adjustment the voltage has been impressed upon the inputbranch ot the circuit, thereby rendering it impossible to detect thetransmission of currents through the balancing network unless the spacecurrent-s were absent, because of the forward or relay action oi':l theaudion space currents which transmitted the input currentsto the outputbranch in ampliiied torni and thus obscured theA currents transmittedthrough the balancing network. But when the dfriving voltage isimpressed upon the output branch in accordance With my invention theadjustments may be made as above described, with the audion lilamentheated to its normal temperature, With the normal B- battery potentialapplied to the plate, and with the normal grid bias, that is, 'with allthe normal space currents loiving. This arises l'rom the fact that thereis no appreciable backward action from the output branch upon the inputbranch through the agency oi' the audion space currents. The onlybackward action which -can occur is that through the audion capacities'and through the balancing network, and 'it is the purpose of theadjustment to limit or suppress such action. i

A further step in my process comprises connecting across the inputterminals or at some convenient point in the input branch ot the circuitra detecting or"I indicating means, to ascertain 'when the desiredadjustment has been made. For instance, I may so adjust the circuit thata null point shown by the indicating device," which indicates a completesuppression 'oi' retroactive currents andi complete unidirectionalaction ot the circuit; or I may so adjust it that a desired response otany predetermined character is obtained. C lt. should be understood thatthe above described circuit and method of adjustment are merely typicalof one application ot' my invention, Which is in no Way limited to thisparticular circuit or means et y.balancing the bridge. F or example, theilheatstone bridge in the above circuit might have as its variableelement the mutua-l inductance M23. hiy invention is equally applicableto the neutrodyne type ot circuit above lrei'eri'cd to. l believemyseltl the irst to'have successfully adjusted audion repeater4circuits, under operating conditions, to be unidirectional in action.

In Fig. Il, I have shown diagraimnatically an arrangement of apparatustor carrying out this djustment. A shielded receiver l is connectedacross the input terminals of the audion repeat-er circuit 2 whichis'being adjusted. A shielded radio frequency oscillator and modulator 3is connected across the output terminals ot the repeater circuit 2.rlelephones or other indicatingY means are connected to the receiver l.Modul-ated radio frequency oscillationsot any desired frequency are thusimpressed fbackWard through the audion repeater circuit which isadjusted until the desired response or freedom from response is given`by the indicating means. i

An alternative arrangenient is shown in I? ig. III, in which l. is a`shieldedi receiver connected to telephones as in Fig. II, and in which 3is a shielded radio frequency oscillator. Instead of modulating thisoscillator as in the previous arrangement, I employ a separateheterodyne oscillator to modulate the oscillations after they havepassed through the audion repeater circuit.

My invention is not confined to a radio frequency balance. I may impressoscillae tions of any suitable frequency backwards through the circuitwhich is to be adjusted. The ciicuit may thus be balanced at anyfrequency for which it is designed, including audio frequencies and alsointermediate frequencies. l

Any oscillator capable of producing oscillations over thedesired-frequency range is suitable for use at I have us-ed an audionoscillator with an inductively coupled oscillation circuit. I may employany form of receiver, but have found satisfactory one having a detectorand tivo, stages of audio amplification Working into telephones. It isof the utmost importance for the best results that both the oscillatorand the receiver together With all their batteries should be thoroughlyshielded so that there may be no direct action between them. Thus anyresponse obtained in the receiver is f necessarily due to oscillationsforced backward through the audion repeater circuit which is beingadjusted.

Arrangements other than those shoivn in Figs. II and III Will occur tothose skilled in the art. For example, the modulation may be supplied byan antenna circuit receiving oscillations from a separate source. If anindicating means other than telephones is used in the process ofobtaining a balance at radio frequencies, it may be unnecessary tomodulate the radio frequency oscillations. Such Would be the caie if agalvanometer were employed.

It should be understood that my invention is not limited to theadjustment of a single audion stage but may be applied to the adjustmentof any audion repeater circuit comprising one or more audion tubes, andthat either a separate adjustment of each stage or a simultaneousadjustment of all the stages falls Within the scope of my invention.Moreover, it is not limited to any particular circuit or type of circuitand applies broadly to any circuit susceptible of adjustment to produceunidirectional action.

l. In the process of adjusting an audion repeater circuit to limitinteraction between the input and output branches of said eircuit, thesteps which comprise impressing an oscillatory voltage upon the outputbranch of said circuit and receiving resulting currents in the inputbranch of said circuit.

2. The method of adjusting an audion repeater circuit to limitinteraction between the input and output branches of said circuit Whichcomprises impressing an oscillatory voltage upon the output branch andvarying a reactive element of said circuit to produce currents of acertain predetermined character in the input branch of said put branchto the input branch of said circuit, and detecting said current in theinput branch.

5. The method of adjusting an audion repeater circuit to beunidirectional in action which comprises impressing` radio frequencyoscillations upon the output terminals of the circuit, connecting anindicating means across the input terminals of the circuit, andadjusting the circuit until the indicating means gives a desiredresponse.

6. The method of adjusting an audion repeater circuit to beunidirectional in action which comprises impressing radio frequencyoscillations uponthe output terminals of the circuit, connecting anindicating means across the input terminals of the circuit, andadjusting the circuit until the indicating means gives a minimumresponse.

7. The method of adjusting an audion repeater circuit to beunidirectional in action which comprises impressing radio frequencyoscillations upon the output terminals of the circuit, connecting anindicating means across the input terminals of the circuit, andadjusting the circuit until the indicating means indicates a null point.

8. The method of adjusting an audion repeater circuit to beunidirectional in action which comprises impressing modulated radiofrequency oscillations upon the output terminals of the circuit,connecting an indicat ing means across the input terminals of thecircuit, and adjusting the circuit until the indicating means gives adesired response.

9. The method of adjusting an audion repeater circuit to beunidirectional in action which comprises impressing radio frequencyoscillations upon the output terminals of the circuit, modulating theseoscillations, connecting an indicating means across the input` terminalsof the circuit, 'and adjusting the circuit until the indicating meansgives a desired response.

10. The method of adjusting an audion repeater circuit containing aplurality 'of audion tubes to be unidirectional in action whichcomprises impressing an 'oscillatory voltage upon the output teru'iinalsolf said circuit, connecting an indicating means across the inputterminals olf said circuit, and adjusting said circuit until theindicating means gives a. response of" a predetermined character.

1l. The method oi adjusting an audion repeater circuit containing auplurality ot' audion tubes to be unidirectional in action whichcomprises impressing an oscillatory voltage upon the output terminals ofsaid circuit, connecting an indicating means across the input terminalsof said circuit, and adjusting said circuit until the indicating meansgives a minimum response.

l2. The method oi adjusting an audio-n repeater circuit containing aplurality ot' audion tubes to be unidirectional in action whichcomprises impressing an oscillatory Voltage upon the output terminals ofsaid circuit, connecting an indicating means across the input terminalsof said circuit7 and adjusting said circuit until the indicating meansindicates anull point,

13. Apparatus for adjusting an audion repeater circuit to bevunidirectional in action, comprising, in combination, means forimpressing an oscillatory voltage upon the output terminals of saidcircuit and indicatingmeans connected to the input terminals of saidcircuit.

la. Apparatus for adjusting an audion repeater circuit to beunidirectional in action7 comprising, in combination, a radio frequencyoscillator connected to the output -terminals of said circuit, means:t'or modulating the resulting radio frequency oscilla#V tions, anddetecting means connected to the input terminals of said circuit.

In testimony Whereol', I atii'x my signature. Y

vLEWIS M. HULL.

